This invention relates to improved means for physically cleaning ion selective membranes, such as reverse osmosis and electrodialysis membranes, without the necessity for using extensive chemical cleaning agents. This is accomplished via ultrasonic vibration of the liquid lying adjacent to the membrane to be cleaned or by vibration, below the threshold of cavitation, transmitted within or adjacent to the membrane. Positioning and/or movement of the ultrasonic transducers prevent standing waves which can otherwise ultimately destroy the membrane. The vibration produced is designed to scour off contaminants on the membrane surface into the bulk feed solution.
The reverse osmosis (RO) process is the most significant water treating process to be discovered within the past 50 years and is gaining more and more uses with commercial industries. Large sums have been spent in research and development efforts with emphasis on developing new membranes with greater water purifying capability. Chemical cleaning of the membranes is the only parameter holding back universal application of the reverse osmosis process. Physical cleaning of such osmosis systems, without destruction of the membranes, has long been needed.
A host of organic and inorganic compounds collect on the surface of ion selective membranes from the passage of contaminated feed streams across the membrane surface. These contaminants block the water permeating surface of the membranes and thereby reduce the produce flux of operations such as reverse osmosis. This product flux reduction is called fouling and this effect must be reduced by periodic cleaning to prolong the membrane's useful life. By reducing the effect of fouling, the economics of water demineralization (or desalination) is greatly enhanced.
Chemical cleaning requires flushing the membrane modules for periods of 10 to 60 minutes. This process also requires purified water (which reduces product water production), skilled personnel to administer chemicals in proper dosages, and a constant chemical supply. This chemical supply takes two forms: cleaning chemicals and pre-treatment chemicals. Pre-treatment includes chemical coagulation, and sedimentation or filtration. Using conservative estimates for coagulation chemicals (2 grains/gal Fe Cl.sub.3,3 grains/gal Ca CO.sub.3) 6,500 pounds/year are required to treat 25,000 gal/day of water. In addition to the pre-treatment chemical expenditures, 4,000 pounds/year of chemicals are necessary to clean reverse osmosis membranes for 25,000 gal/day water production. This combined chemical expenditure is 10,500 pounds/year and it is this logistic, and operational and maintenance burden which has kept membrane processes from many commercial and other applications.
There is a reverse osmosis system that involves cavitation to facilitate the passage of water through a semi-permeable membrane in a reverse osmosis process and uses a combination of high frequency vibrational energy and high pressure to oppose the natural osmotic pressure. However, this system does not provide means to prevent destruction of the membrane from the destructive effects of cavitation and high pressure other than using stronger types of membranes preferably reinforced with glass fibers, or powdered metal to strengthen the membrane and increase its useful life against cavitation destruction.